This invention relates to electrical apparatus for accurately driving a load in proportion to a source voltage, and more particularly relates to apparatus for indicating the voltage generated by a signal generator, such as a Hall generator, a strain gauge bridge, or a Wheatstone bridge.
Referring to FIG. 1, the prior art typically has used a differential (difference) amplifier in order to drive a grounded load L from an ungrounded or floating signal generator G. The differential amplifier typically comprises an operational amplifier OP1 having an inverting input II and a non-inverting input NI. Operational amplifier OP1 is connected as a differential amplifier by means of input resistors R1-R3 and a feedback resistor R4. In order to amplify the difference in voltage produced by generator G and to reference that voltage to ground potential, the equation R.sub.4 /R.sub.1 = R.sub.3 /R.sub.2 must be satisfied as closely as possible, where R1-R4 equal the values of resistors R1-R4, respectively, in ohms. If a modern operational amplifier is employed, the common mode rejection characteristic of the stage, that is, the extent to which the amplifier output is independent of the common mode input voltage, is limited by the accuracy with which the foregoing equation is satisfied.
For some applications, such as the amplification of a Hall generator output voltage, or a thermocouple output voltage, a common mode rejection of at least 100 decibels, 100,000 to 1, is generally required. To achieve this degree of common mode rejection, the resistors would have to be matched to accuracies better than 0.001% over a wide range of temperatures and frequencies and for long periods of time. Such as approach would require an expensive resistor network, possibly located in a thermally-stabilized chamber.
It has been discovered that the deficiencies of the prior art can be overcome by combining non-differential and differential amplifier stages which are referenced to the signal generator and load in a novel manner. The signal generator generates a source voltage between first and second generator outputs. A non-differential gain stage generates an amplified voltage between the second generator output and the gain stage output which is proportional to the source voltage. The amplified voltage then is routed through a differential stage for generating a rereferenced voltage between the differential stage output and a first terminal of the load which is proportional to the amplified voltage from the non-differential stage and which is substantially independent of any offset voltage between the second generator output and the first terminal of the load. By coupling at least a portion of the rereferenced voltage to the load, the load can be driven in proportion to the source voltage even though the offset voltage may vary over a substantial range. By using this technique, common mode rejection of about 100,000 to 1 can be achieved by using conventional 1% resistors, and the need for a thermally-isolated environment is eliminated.
It also has been discovered that the source voltage can be measured and displayed on an indicator with improved precision by using this invention. According to this feature, the load comprises an amplifying stage which drives the indicator. Adjustable resistors can be employed in order to add gain adjustment and zeroing capability to the system. By using this arrangement, the meter can be driven in various ranges and can be zeroed to accommodate different DC offsets of the non-differential gain stage, differential stage or signal generator.